Suspended animation is the slowing or stopping of life processes by exogenous or endogenous means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use of this process has led to the developing science of cryonics. Cryonics is another method of life preservation but it cryopreserves organisms using liquid nitrogen that will preserve the organism until reanimation. Two-celled embryos have been kept in suspended animation for as long as 13 years.
Placing astronauts in suspended animation has been proposed as one way for an individual to reach the end of an interstellar or intergalactic journey, avoiding the necessity for a gigantic generation ship; occasionally the two concepts have been combined, with generations of "caretakers" supervising a large population of frozen passengers.
Since the 1970s, induced hypothermia has been performed for some open-heart surgeries as an alternative to heart-lung machines. Hypothermia, however, provides only a limited amount of time in which to operate and there is a risk of tissue and brain damage for prolonged periods.
In June 2005 scientists at the University of Pittsburgh's Safar Center for Resuscitation Research announced they had managed to place dogs in suspended animation and bring them back to life, most of them without brain damage, by draining the blood out of the dogs' bodies and injecting a low temperature solution into their circulatory systems, which in turn keeps the bodies alive in stasis. After three hours of being clinically dead, the dogs' blood was returned to their circulatory systems, and the animals were revived by delivering an electric shock to their hearts. The heart started pumping the blood around the frozen body, and the dogs were brought back to life.
On 20 January 2006, doctors from the Massachusetts General Hospital in Boston announced they had placed pigs in suspended animation with a similar technique. The pigs were anaesthetized and major blood loss was induced, along with simulated - via scalpel - severe injuries (e.g. a punctured aorta as might happen in a car accident or shooting). After the pigs lost about half their blood the remaining blood was replaced with a chilled saline solution. As the body temperature reached 10 °C (50 °F) the damaged blood vessel were repaired and the blood was returned. The method was tested 200 times with a 90% success rate.
From May 2014, a team of surgeons from UPMC Presbyterian Hospital in Pittsburgh plan to try the above method in gunshot victims (or those suffering from similar traumatic injuries). The trials will be done on ten such severely wounded patients and compared with ten others in similar situation but who had no access to the above method. 
An article in the 22 April 2005 issue of the scientific journal Science reports success towards inducing suspended animation-like hypothermia in mice. The findings are significant, as mice do not hibernate in nature. The laboratory of Mark B. Roth at the Fred Hutchinson Cancer Research Center in Seattle, Washington, placed the mice in a chamber containing 80 ppm hydrogen sulfide for a duration of 6 hours. The core body temperature of the mice dropped to 13 degrees Celsius and metabolism, as assayed by carbon dioxide production and oxygen use, decreased 10-fold. They also induced hypoxia on nematode embryos and zebrafish embryos, placing them in suspended animation for hours, and then re-animating them simply by returning the oxygen to the embryos.
Massachusetts General Hospital in Boston announced they had been able to hibernate mice using the same method. Their heart rate was slowed down from 500 to 200 beats per minute, respiration fell from 120 to 25 breaths per minute and body temperature dropped to 30 °C (natural: 39 °C). After 2 hours of breathing air without hydrogen sulfide the mice returned to normal. Further studies are needed to see if the gas had damaging effects on the brain, considering the effect of hydrogen sulfide on the body is similar to hydrogen cyanide; it does not slow the metabolic rate but rather inhibits the transfer of energy within the cell via ATP.
Experiments on sedated sheep and partially ventilated anesthetized pigs have been unsuccessful, suggesting that application to large mammals may not be feasible. In any case, long term suspended animation has not been attempted.
There are many research projects currently investigating how to achieve "induced hibernation" in humans. This ability to hibernate humans would be useful for a number of reasons, such as saving the lives of seriously ill or injured people by temporarily putting them in a state of hibernation until treatment can be given.
Actual and anecdotal cases of suspected human hibernation or states similar to hibernation exist in the literature:
- Anna Bågenholm, a Swedish radiologist who survived 40 minutes under ice in frozen lake in state of cardiac arrest and survived with no brain damage in 1999
- Mitsutaka Uchikoshi, a Japanese man who survived the cold for 24 days in 2006 without food or water when he fell into a state similar to hibernation
- Paulie Hynek, who, at age 2, survived several hours of hypothermia-induced cardiac arrest and whose body temperature reached 17.8 C (rounded up)/64 F
- Armando Socarras Ramirez and a number of other wheel-well stowaway flight survivors
- "Longest frozen embryo baby born". BBC News. 6 July 2005. Retrieved 14 January 2009.
- "Triplets born 13 years apart". Times Online. 6 July 2005. Retrieved 24 January 2010.
- Mihm, Stephen (11 December 2005). "Zombie Dogs". The New York Times.
- USA (2013-03-25). "Does the rate of rewarming from profound hypothermi... [J Trauma. 2006 - PubMed - NCBI". Ncbi.nlm.nih.gov. Retrieved 2013-06-23.
- "Doctors claim suspended animation success". The Sydney Morning Herald. 20 January 2006. Retrieved 10 October 2006.
- "Left between life and death: First 'suspended animation' trials set to begin in bid to buy time for stabbing and gunshot victims.".
- Blackstone, E.; Morrison, M.; Roth, M. (2005). "H2S induces a suspended animation-like state in mice.". Science 308 (5721): 518. doi:10.1126/science.1108581. PMID 15845845.
- "Gas induces 'suspended animation'". BBC News. 9 October 2006. Retrieved 10 October 2006.
- Haouzi P, Notet V, Chenuel B, Chalon B, Sponne I, Ogier V, Bihain B (2008). "H2S induced hypometabolism in mice is missing in sedated sheep". Repiratory Physiology &Amp; Neurobiology 160 (1): 109–115. doi:10.1016/j.resp.2007.09.001. PMID 17980679.
- Li, Jia; Zhang, Gencheng; Cai, Sally; Redington, Andrew N (January 2008). "Effect of inhaled hydrogen sulfide on metabolic responses in anesthetized, paralyzed, and mechanically ventilated piglets" (Subscription required). Pediatric Critical Care Medicine 9 (1): 110–112. doi:10.1097/01.PCC.0000298639.08519.0C. PMID 18477923. Retrieved 23 March 2008.
H2S does not appear to have hypometabolic effects in ambiently cooled large mammals and conversely appears to act as a hemodynamic and metabolic stimulant.
- New Hibernation Technique might work on humans | LiveScience at www.livescience.com
- Race to be first to 'hibernate' human beings - Times Online at www.timesonline.co.uk
- Japanese man in mystery survival at BBC News
- Eleva boy’s story part of national tour to honor Mayo Clinics 150 years Mayo Clinic
- Mark Roth: Suspended animation is within our grasp
- Pitt scientists resurrect hope of cheating death
- Suspended Animation: Bringing Back the Dead?
- "Stuck Pig" article at Wired.com
- Hydrogen sulfide does not induce hypometabolism in sheep
- Suspended Animation Technology arrives
- Laboratory of Mark B. Roth at the Fred Hutchinson Cancer Research Center